A person that is there in body only.
Someone who is so unattached or unaware of their own life that their involvement has little or no impact on the outcome of events.

"Charlie, don't you get it? I can't feel that. It's sweet and everything, but it's like you're not even there sometimes. It's great that you can listen and be a shoulder to someone, but what about when someone doesn't need a shoulder. What if they need the arms or something like that? You can't just sit there and put everybody's lives ahead of yours and think that counts as love. You just can't. You have to do things."
Charlie is an empty seat.